Water Cycle
Water can exist either as a solid (ice), a liquid (water), or a gas (water vapor). Water
on the on surface of Earth is constantly changing between these three states. Ice can
change to become water or water vapor. Water can change to become ice or water vapor.
Water vapor can change to become ice or water. These continuous changes in state create a
cycle of repeating events.

Water continually circulates between
the surface of Earth and its atmosphere in what is called the hydrologic or water cycle.
Responding to heat energy from the Sun, water in oceans, lakes, swamps, rivers, plants and
even in your body can turn into water vapor. Water vapor in the atmosphere condenses as it
cools to form clouds. Once the droplets of condensed water vapor are too heavy to remain
in the atmosphere, they fall to Earth as precipitation. Rain, snow, sleet, fog and dew are
all forms of precipitation.

After the precipitation reaches the surface of Earth, it does one of
four things. It can either be absorbed by plants, percolate through the soil to become
ground water, run off the surface into streams and rivers--becoming surface water and
eventually flowing into the oceans, or evaporate.

Water is a major force in the sculpting of Earth's surface and is an
important means of transporting the energy that drives atmospheric circulation. Although
water covers three-fourths of the surface of Earth, water represents a relatively small
percentage of Earth's total volume--a 3-foot diameter model of Earth would have only one
cup of water. Thus, water is actually relatively very scarce.